1. Field of the Invention
This invention relates to a method of forming a dielectric film suitable for applying to a semiconductor device having a capacitance element. More specifically, this invention relates to a capacitance element and a semiconductor device each using a dielectric film and further relates to methods of manufacturing them.
2. Description of the Related Art
Recent semiconductor devices have significantly increased in capacity and, in the DRAM (Dynamic Random Access Memory) field, large-capacity memories of 1 Gbit have been put to practical use. DRAM cells each normally comprise one transistor and one capacitor. Charges representing information are stored into the capacitors through the transistors or information in the form of charges stored in the capacitors is read through the transistors. Each capacitor comprises two electrodes, i.e. a lower electrode connected to a diffusion layer electrode of the transistor and an upper electrode commonly connected to a reference potential, and further comprises a capacitor dielectric film disposed between those electrodes.
In the large-capacity memory, following the reduction in size of memory cells, the occupation area of a capacitor portion is also reduced in each memory cell. However, in the memory such as the DRAM, since charges in the capacitors are used as stored information, each capacitor is required to have a capacitance greater than a certain value in order to ensure the stable memory operation. For achieving the capacitance greater than the certain value in the reduced memory-cell area, it is necessary to reduce the thickness of a capacitor dielectric film or to use a high-permittivity material as a capacitor dielectric film. For example, in the current state, a film thickness of 1 nm or less is required in terms of a silicon oxide (SiO2) film. For this purpose, practical use is made of a dielectric film, such as an AlOx (aluminum oxide; relative permittivity: about 9) film or a Ta2O5 (tantalum pentoxide; relative permittivity: about 50) film, having a higher permittivity than a SiO2 (silicon oxide) film and a Si3N4 (silicon nitride) film which have hitherto been used. Further, practical use of a strontium titanate (hereinafter also referred to as “STO”) film having a relative permittivity exceeding 100 is also now under review.
For example, in Patent Document 1 (Japanese Unexamined Patent Application Publication (JP-A) No. 2004-146559), polycrystalline ruthenium is used as a lower electrode of a capacitor and, as a dielectric film thereof, an amorphous strontium titanate film is formed by chemical vapor deposition (CVD) and then is crystallized into a single-crystal strontium titanate film by a heat treatment.
There is a problem that although a crystalline strontium titanate film exhibits a high relative permittivity, it cannot be practically used because of its large leakage current along crystal grain boundaries.
As a countermeasure therefor, there is proposed a technique using a composite structure of a polycrystalline STO film and an amorphous STO film as a dielectric film, for example, in Patent Document 2 (Japanese Unexamined Patent Application Publication (JP-A) No. 2003-282717) or Patent Document 3 (Japanese Unexamined Patent Application Publication (JP-A) No. Hei 09-202606).